126 AMERICAN MARINE CONCHOLOGY. 
Subgenus CYRTOPLEURA, Tryon. 
Monog. Pholadacea, 73. 1862. 
Margin of the valves emarginate anteriorly, making a short, 
wide hiatus. 
2. P. TRUNCATA, Say. Figs. 260, 261. 
Journ. Philad. Acad., ii. 8321. 1822. 
Shell subpentangular; anterior obtusely rostrated, wedge-shaped 
in the middle; posterior margin broadly truncated at the tip; 
valves transversely wrinkled, crossed by striz, muricated ante- 
riorly by small erect scales, which form ribs from the beak to 
basal margin. 
Vertical axis 1, transverse 2.5 inches. 
Whole Coast from Sable Island to W. Indies; also W. Coast of S. A. 
Genus DACTYLINA, Gray. 
Proc. Zool. Soc., 187. 1847. 
In the typical form of this genus the nuclei of the dorsal valves 
are situated at their outer margins, posterior to the centre; the 
valves are much emarginate anteriorly, forming a short, wide 
hiatus. I have characterized as a subgenus, a form of which our 
American species is the type, as follows :— 
Subgenus GITOCENTRUM, Tryon. 
Monog. Pholadacea, 75. 1862. 
Nuclei of the dorsal valves anterior, situated nearer the inner 
margin; dorsal plates marked by radiating lines; valves not 
emarginate anteriorly, but regularly rounded; hiatus long and 
narrow. 
1. D. Camprcuensis, Gmelin. Figs. 262, 263. 
(Pholas.) Syst. Nat., 3216. 1790. 
Pholas oblongata, Say, Journ. Philad. Acad., ii. 820. 1822. 
Pholas Candeana, Orb. Moll. Cuba, 215, t. 25, f. 18, 19. 
Pholas Chiloensis, King, Zool. Journ., v. 334. 1832. 
Shell thin, white transversely, much elongated ; basal and hinge 
margins nearly parallel ; anterior and posterior margins rounded ; 
valves transversely and longitudinally striated, the striz muri- 
cated and elevated on the anterior side into costs, which are 
more prominently and densely muricated; hinge callus polished, 
